1. Field of the Invention
The present invention relates to a driving circuit of an active matrix method in a display device.
2. Discussion of the Related Art
Recently, various display devices such as an LCD device, a PDP device, an FED device and an EL device have been studied with development of flat display devices. These flat display devices are classified into two according to a driving method, a passive matrix method and an active matrix method. At this time, it is required to use a higher level of current in the passive matrix method than the active matrix method.
Accordingly, in current driving methods of the LCD device and the PDP device, since greater current level is required with increasing the number of pixel, the passive matrix method is more efficient.
Meanwhile, in current driving methods of the FED and EL devices, it is regarded that the active matrix method is more efficient than the passive matrix method since it is required to use the higher level of current in the passive matrix method than the active matrix method even though a line time is equal.
FIG. 1 is a circuit diagram of a driving circuit according to a related art active matrix method.
As shown in FIG. 1, the driving circuit includes a scan line SEL, a data line DATA, a switch P1, a capacitor Cs, a driving transistor PO, an OEL and a positive power supply VDD.
At this time, the scan line SEL selects a pixel for driving, and the data line DATA applies a voltage to the pixel. The switch P1 is served as an active device to control data input according to a signal of the scan line, and the capacitor Cs stores electric charges selected according to the voltage applied to the data line. Next, a voltage is input to the driving transistor PO by the electric charges stored in the capacitor Cs, and then the driving transistor PO applies a current to the OEL. The OEL emits light by the current applied from the driving transistor PO, and the positive power supply VDD supplies a power to the capacitor Cs and the driving transistor PO.
An operation of an active matrix method in a related art display device will be described in detail.
First, the pixel driven by the scan line SEL is selected, and then the pixel for driving is turned on by the switch P1. Then, a control voltage, in which a gray is controlled, is applied to the pixel for driving through the data line.
At this time, the control voltage stored in the capacitor Cs, simultaneously, drives the driving transistor PO to make the GEL emit lights.
After the scan line is disabled, the driving transistor PO is driven by the voltage stored in the capacitor Cs to maintain one frame until the next select time.
However, since threshold voltages of the driving transistors used in the display device are different, the driving current for driving the OEL selected is not constant even though an equal driving voltage is applied to each driving transistor.
That is, each OEL emits different luminance according to deviation of the threshold voltages of the driving transistors.
To decrease the luminance deviation of the OEL according to the deviation of the threshold voltages of the driving transistors, it is required to constantly apply the driving current for driving the OEL without regard to the deviation of the threshold voltages of each driving transistor.
The deviation of the threshold voltages of the driving transistors is necessary consequence in fabricating process steps of the display device. Therefore, the luminance deviation of the pixels has to be compensated by detecting luminance of each pixel, however, it is hard to effectively compensate the luminance deviation.
Also, in the related art driving circuit, if a margin of the control voltage according to level of the driving current is small, it is hard to obtain desired luminance.